Unit magnification optical system for angular measuring instruments



Feb. 27, 1968 ANGULAR MEASURING INSTRUMENTS Filed July 2. 1964 JOHN M. SCHWARTZ INVENTOR fimz United States Patent 3,370,903 UNIT MAGNIFICATION OPTICAL SYSTEM FOR ANGULAR MEASURING INSTRUMENTS John M. Schwartz, Irondequoit, N.Y., assignor to Bausch & Lomb Incorporated, Rochester, N.Y., a corporation of New York Filed July 2, 1964, Ser. No. 379,992 2 Claims. (Cl. 350-202) ABSTRACT OF THE DISCLOSURE lateral movement of said cube results in relative motion of said scale and reticle.

This invention relates to a novel optical system and more particularly to a unit magnification system for use in a microscope or the like.

Angular measuring instruments of the type having circular rulings and means sensing the positions of those rulings have recently been developed. The recently developed instruments have the potential to detect rotational movement of extremely small magnitudes. Such devices should detect angular milliseconds when properly equipped with precision optical systems. For example, the devices which are disclosed and claimed in the copending application of Kreckel et al., 275,540, filed Apr. 25, 1963 and assigned to the same assignee as the present invention are designed to detect and indicate minute rotational displacements. Accordingly, they have created a demand for a precision optical system.

Measuring systems such as those disclosed in the aforementioned application include an imaging system for superimposing an image of a reticle and a scale, so that, an angular displacement of a portion of a cycle (a'cycle being one transparent and one opaque area) progressing from one reticle to another may be accurately measured. The imaging means for optically imaging areas of the scale or to image different ones of the reticles onto different portions of the scale to thereby regulate the light intensity are utilized in those systems. The optical systems disclosed and claimed herein have been designed particularly for the requirements of those instruments disclosed and claimed in the aforementioned application. The particular problems relating to those devices have been overcome to a relatively high degree by the system disclosed and claimed herein.

For example, the lens systems image each of eight areas of a scale onto each of eight reticles at unit magnification. The spacings between the scale divisions are relatively small, i.e., in the range of two seconds of arc and interpolation to 100 seconds of are for a 500 millimeter diameter circular scale are obtainable with the present optical system. The aberrations of the optical system have been minimized while the long conjugate distances were in- "ice creased to infinity. Further the third order or Seidel aberrations were corrected for the particular application.

Briefly, the present invention comprises a pair of objec tives placed back to back with their long conjugates at infinity to thereby obtain a unit magnification system. The separation of the objectives is such that they are separated by a distance which is nearly equal to twice the exit pupil distance from the lens vertex nearest to the long conjugate. A beam splitter prism is inserted between the two objectives. Each of the objectives include the following axially aligned elements I through V. Elements I through V are taken from the prism.

The element I defines a double convex lens having the longer radius on the side of the prism. A second element II comprises a convex concave lens with the concave surface having a radius which matches the shorter radius of the element I and is cemented thereto to form a compound lens. A plano convex singlet makes up the element III and is airspread from the doublet 1-H and a doublet IV, V. The doublet IV, V includes a double convex lens IV with its shortest radius closest to the prism. The element V defines a plano concave lens wherein the radius of the concave surface matches the rear convex surface of the element IV.

The lens system according to the present invention will now be described in connection with the accompanying drawing; in which:

The single figure of the drawing is an axial section of a lens system according to the present invention.

The lens system shown in the drawing comprises a pair of objective lenses 10 and 10' and a beam splitter 11 separating the two objectives. The objectives 10 and 10' are disposed in a back to back relationship with their long conjugates at infinity. Minimum aberrations were maintained in the systems by constructing each of the two objectives 10 and 10' including the elements I through V and I through V' respectively in accordance with the constructional data shown in Table A.

TABLE A wherein R to R and R to R are the radii of the successive lens surfaces t to t and 1 to t are the axial thicknesses of said elements therebetween and S to S and S to S, are the axial spacings.

The minus sign used with certain of the R values means that such a surface is concave toward the beam splitter cube 11.

For reasons such as resolving power, depth of focus, working distance and exit pupil location, the system uses two modified 0.40 NA .20X type microscope objectives.

The two objectives 10 and 10' are airspaced from each other by a distance of between 8.1 to 9.9 mm. A beam When the pair of objectives are placed back to back in a symmetrical manner, the coma is substantially reduced. Lateral color and distortion are also reduced by the symmetry of the system. For example, the aberration coeflicients for the novel combination of elements disclosed and splltter cube 11 of between 8.4 and 9.6 mm. rs disposed claimed herein are as follows: between the objectives and permits observation of the Aberration type: Coefficients superposition of the scale and retrcle images. Spherical 007159 It is also desirable to construct the lens system in Coma +:0oo070 acclorglange with the specific constructional data set forth m Astigmatism m a e Petzval curvature .005343 TABLE B Distortion +.o0oo4s 1.517o n1" n1 1 519o Longitudinal color .001238 1.7100 n,',n, 1.73o0 1r Lateral 1.5790 n n 1.5990 The two objectives and 10' according to a preferred -l.5630 n n 1.5850 embodiment of the invention are separated by a distance 1.5650 n ',n l.S850 of 9.6 mm. and a beam splitter cube 11, 9 mm. square, 54.6 w v 64.6 and preferably having an index of 1.5725 is disposed in 24.3 v v 34.3 the airspace which separates the objectives. The objec- 56.2 v r 66.2 tive according to the preferred embodiments conform to 52.4 v v 62.4 the following specific constructional data shown in Table 36.4 y r 46.4 C.

TABLE C Element Radius Thickness or Index of Re- Abbe Spacing fraction nu Number 9 Sr, Sr=.3 Br, R1 =50.00 I, I 1', tr =33! flr', Pd-5180 Ir, n 5.6

-Rs', Bj =8.90 n' n u z m 172m 29 S1, S1=O.64 n1 m 1' 2 a90 n =1 ssoo =61 2 R". R. Plano a 1 fl: r: r;

s=', s1=1.e7 R7, R7 =8.50 IV, IV h, 4 =3.77 m, 1l|=1-5730 n, I|=57A R R =12.77 v' v r' t 408 15150 1 Rm Rm man s m m n a"! .4

s4, s,=2.es

wherein n to n, and n;' to n, are the indices of refracwherein R to R and R to R are the radii of the tion and 9 to and 1 to 1 are the Abbe numbers of lens surfaces of said elements, t to t are the axial thickthe elements I through V, I through V respectively. 60 nesses thereof, 8; to S and S to S are the axial spac- Minimum aberrations were maintained or minimized in ings therehetween, n to u and n to u are the indices the lens systems according to the present invention while of refraction and r; to 1/ and 7 to 1 are the Abbe numsetting the long conjugate distances at infinity in order to bers of the elements I through V and I through V' reobtain optimum performance of the system. For example, spectively of said two objectives. the third order on Seidel aberration coefli cients for a While the invention has been described in connection single objective are listed below for object distance equal with a specific application it may be modified or embodied to infinity. in other forms without departing from the scope of the C flic-ent appended claims. gg igg g i 2 2 What is claimed is:

Coma 00 257 7 1. An optical system including a pair of similar objec- Astigmafism 0()()926 tives disposed in a back to back relationship with their Petzval curvature -.003193 l ng conjugates at infinity and a beam splitter cube dis- Distortion -.000870 posed between said objectives, each of said objectives in- Longitudinal color .000737 eluding five elements I through V and l' to V' respec- Lateral color .001714 tively, a first element I defining a double convex lens having the longer radius on the side of the prism, a second element II comprising a convcxo concave lens having a radius equal to the adjacent radius of element I and in contact therewith, a plane convex singlet III airspaced from the element II, and a doublet including elements IV and V airspaced from the singlet III, the doublet IV, V including a double convex lens IV having its shortest radius closest to the prism, the element V defining a piano-concave lens, said elements conforming substan. tially to the following constructional data:

wherein R to R and R to R are the radii of the lens surfaces of the elements, t to i and I to 1 are the axial thicknesses, S to S and S to S are the axial spacings, n to 12 and n to n are the indices of refraction and v to 9 v to 11 are the Abbe numbers of the respective elements IV to I to V, the minus signs used with certain R values meaning that such a surface is concave toward said beam splitter cube.

2. An optical element according to claim 1 in which 10 the beam splitter cube is approximately 9 mm. square and has an index of refraction of about 1.5725.

Element Radius Thickness or Index of Re- Abb Spacing fraction u Number 1 S1, S1=.3 R1, R1 =50.00 I, I R I R 8 90 t t1 =3.94 7L1, 7h=l.0180 v v1=59.fi

z, z I 11' u R3 z, z, =l97 n n 1-2oo 9a -R.', -R. =32.11 P

R R 1480 SFO'M s III, 111 1 R H 11,21 =a.9o m, n;=1.5890 va',n=61.2

5 8H0 R I SJ, S3=L67 1v IV RPM) tt =a71 1: 15'30 514 Rs Rs 12." 4 4 4 7H- I I4 vr- -Ra', -R =12.77 V',v n R P1 is',l5 =4.08 m,1z;=1.5750 y5',r5=41.4

m, m= ano References Cited UNITED STATES PATENTS 1,945,977 2/1934 Oswald 350175 X DAVID H. RUBIN, Primary Examiner. JEWELL H. PEDERSEN, Examiner. J. K. CORBIN, R. J. STERN, Assistant Examiners.

UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,370 ,903 February 27 1968 John M. Schwartz It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 TABLE A, line 3 thereof, "8 .0l R -R 9. 79"

should read 8.Ol -R ,-R 9.79 same TABLE A, line 4 thereof, "28.90 R ,-R 3S.32" should read 28.90 R -R 35.3Z Column 3, TABLE B, line 4 thereof, "1.5630 n n l.5850" should read l.5630 n ,n l.5830

Signed and sealed this 25th day of November 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

